You might have noticed that objects like wood, hair, clothes, sand, paper etc appear dark when wet than when dry. All such objects that get dark when wet have a similarity. They are porous objects. Any material having pore spaces or very minute holes that are invisible to our eyes us said to be porous. These pores also called voids are filled with either liquid or gas. All those porous objects mentioned above have air (gas) within their pores.
Natural substances like rocks, soil and many man-made substances such as cements and ceramics are considered porous media. When light passes through these kinds of objects, it will hit the object, refract into it (travel through the medium) and then refract back out of it.
Coming back to our earlier discussion, the pores get filled with water when the porous substances become wet. Water has a higher index of refraction (1.33) compared to air (1.00029). This means light is bent more when it travels by water compared to air. Because the tiny reflecting surfaces are now filled with water, they cease to reflect light back to the observer. This makes the wet objects appear dark compared to dry ones. Thus wet objects tend to become more transparent and less reflective.